The Democratic-Republican Party made significant gains during this election cycle, which helped to usher in what is known as the Era of Good Feelings under President James Monroe, who was elected that year. The Federalist Party was in a state of collapse, in part because of the secessionist doctrine espoused by some party members from New England at the Hartford Convention of 1814–15. This created an almost treasonous image of the Federalist party outside its base in urban New England. The War of 1812 concluded in 1815 with a feeling of national pride, since the small American military had fought the much more powerful British forces to a stalemate. The end of the war and the foolhardy posturing of New England Federalists led voters to rally around the dominant Democratic-Republicans and usher in a period of nonpartisan, consensus governance, despite the remnants of party divisions.

The proportion of seats held by the Federalist party in the House of Representatives fell to less than a quarter. Even at that, the election of 1816 gave them the highest proportion of seats that they were ever able to secure before the national party ceased to function as of 1824. In the same period, the Democratic-Republicans enjoyed majorities never again approached by any American political party until the era of Reconstruction in the late 1860s.

The States of Mississippi and Illinois were admitted during the 15th Congress, adding one seat each.[1][2] Mississippi was represented for most of both sessions, while Illinois was only represented during the 2nd session.

John Forsyth (DR) resigned on November 23, 1818, after being elected to the Senate. A special election was held on January 4, 1819, to fill the vacancies left in both the remainder of the 15th Congress and in the 16th Congress to which he had been re-elected before his resignation.

Mississippi was admitted as a state of the Union on December 10, 1817.[4] from the western half of the former Mississippi Territory (the eastern half became Alabama Territory) It elected its first representative to Congress in 1817.

There were four territories with the right to send delegates during at least part of the 15th Congress. Mississippi Territory only existed during the first few months of the 15th Congress, but did not elect a delegate, since it was admitted to the Union as a state a few days into the 1st Session of the 15th Congress. Illinois Territory also only existed during the 1st Session, as it was admitted to the Union as the State of Illinois on December 3, 1818. Alabama Territory was formed from the eastern half of Mississippi Territory, and its first (and only) delegate was seated on March 9, 1818.[4]

There were two elections held for the delegate from Missouri Territory. The first was contested by Rufus Easton on the grounds of electoral fraud. This election was declared void, and a second election was held on August 4, 1817.[4] It was won without controversy by John Scott, who took his seat on December 8, 1817.

^ abcdFigures are at variance with both Dubin (p. 62, which includes "140 Republicans, 1 Independent Republican, and 2 vacancies", in addition to "41 Federalists"), and Martis (p. 84) and "Party Divisions of the House of Representatives* 1789–Present". Office of the Historian, House of United States House of Representatives. (which both report 146 Democrat-Republicans, and 39 Federalists). But Dubin appears to incorrectly list Lewis Williams of NC-13 as a "Federalist" (see pg. 60 and 55, etc.) instead of a "Democrat-Republican", while Martis lists Philemon Beecher of OH-05 as a "Democrat-Republican" instead of a "Federalist", which if accounted for would revise both Dubin's and Martis' totals to 40 Federalists (and therefore 145 Democrat-Republicans).

^A majority was required for election, which was not met in the initial election for 5 districts requiring additional trials to be held on January 27, May 1, July 21, September 29, and December 1, 1817